Re: planetary and atmospheric rotation - origins, direction, etc

Tim Little (tim@xxxxxxxxxxxxxxxxxxxxxxxxxx) writes:
On 2008-02-29, Larry Caldwell <firstnamelastinitial@xxxxxxxxxxx> wrote:
If Venus underwent such a massive collision, where is the debris?
Venus should have one or more moons accreted from the debris.

Slowly rotating planets are very unlikely to have moons. If a moon
orbits faster than the planet, then tidal effects act to decelerate
it. If there was a moon or debris ring, it would long ago have been
decelerated and re-impacted.

E.g. Phobos will impact Mars within less than 100 million years,
because it is orbiting faster than Mars rotates.

Just curious: how unlikely is it that a moon could be formed--or
captured--in a orbit so large that the time scale for deceleration is
comparable to the age of the solar system? (I seem to recall there's at
least one retrograde moon among the outer planets that's not due to fall
any time soon.)

--John Park
.

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